The present invention relates to the manufacture of an optical cable of the type having a core made up of one or a plurality of stranded optical cable elements, each such element being composed of a stable sheath and at least one optical fiber lying loosely therein.
Reference herein to optical fibers is intended to mean fibers of a material suitable for the transmission of light in the infrared, visible, or ultraviolet range or in part of these ranges of the electromagnetic spectrum. Such optical fibers may be encased in a protective sheath in order to protect their surfaces and serve mainly for the transmission of information.
Optical fibers are mechanically, or physically, very sensitive. For that reason they are arranged in protective sheaths either individually or in bundles, for further processing, installation and assembly. Herein, such an arrangement of one or several fibers within a sheath is referred to as an optical cable element, a plurality of which elements are stranded together to form the core of an optical cable. During the stranding operation, the cable elements are subjected to substantial bending and tensile stresses. Similarly, during later installation and assembly of the cable, there occur bending and tensile stresses which act on the light conductor arrangements. This may damage the optical fibers contained in the light conductor arrangements or may reduce their transmission quality.
It is known to protect optical fibers against bending and tensile stresses occurring during the stranding of light conductor arrangements by arranging them loosely in tube-like sheaths which absorb such mechanical stresses. The optical fibers are freely movable within the tube-like sheaths. Such a light conductor arrangement is disclosed, for example, in German Offenlegungsschrift [Laid-open Application]No. 25 56 861.
According to this disclosure, the tube-like sheath is extruded in a spaced arrangement around the optical fibers. The temperature required for this process presents a danger of damage to the plastic sheath around the optical fibers.
A drawback of this known light conductor arrangement is that, after completion of the manufacturing process, it must be wound on drums, and during such a winding process the optical fibers do not remain in the center of the tube but rest against the inner wall of the tube at the side closest to the center of the drum.
When this arrangement of light conductors is then straightened out again for stranding, the fibers are shorter than the tube and are therefore stretched, which often leads to breaks in the fibers.